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Phosphotungstic acid (HPW) molecules anchored in the bulk of Nafion as methanol-blocking membrane for direct methanol fuel cells
Journal of Membrane Science 368 (2011) 241–245
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Phosphotungstic acid (HPW) molecules anchored in the bulk of Nafion as methanol-blocking membrane for direct methanol fuel cells Yan Xiang a,b,∗ , Meng Yang a,b , Jin Zhang a , Fei Lan a , Shanfu Lu a,∗ a b
School of Chemistry and Environment, Beihang University, Beijing 100191, China School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 20 August 2010 Received in revised form 14 November 2010 Accepted 17 November 2010 Available online 23 November 2010 Keywords: Phosphotungstic acid (HPW) Nafion modification Methanol-blocking Direct methanol fuel cell (DMFC)
a b s t r a c t In the present study, a novel composite methanol-blocking polymer electrolyte membrane was prepared by anchoring water-soluble phosphotungstic acid (HPW) in bulk phase of Nafion through interactions between HPW and Cs+ ions. Results of morphology and elemental mapping analysis indicated that this composite membrane had a dense structure with uniformly distributed Cs–HPW clusters. The composite membrane exhibited a rather low methanol diffusion coefficient (P, 0.97 × 10−6 cm2 s−1 ) and also the comparable conductivity (, 3.6 × 10−2 S cm−1 ) contrast with pristine Nafion (, 5.1 × 10−2 S cm−1 ). The selective factor (/P) of the composite membrane was six times higher than that of pristine Nafion. Single cell performance results showed that the maximum power density increased by 26% over the cell performance of pristine Nafion under the same conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) have attracted increasing attention as a promising highly efficient energy conversion power sources for electric vehicles and portable electronic devices due to their low environmental pollutant emissions, simple structural design and convenient fuel storage [1–3]. Proton exchange membrane (PEM) is the “heart” of the DMFC system. The state-of-the-art and most commercialized PEM, the Nafion series membrane, could hardly sustain a satisfactory performance when applying in DMFC, since the methanol could readily migrate from the anode to the cathode through Nafion membrane. This migration results in a mixed potential, thus reducing the open-circuit voltage by as much as 0.15–0.2 V and poisoning the electrocatalysts at the cathode [4–6]. Thus, suppression of methanol crossover in Nafion has attracted intensive attention worldwide, and the approaches include membrane surface modification by depositing palladium and palladium alloys on Nafion membrane [7–9], and using composite membranes, such as sol–gel derived Nafion/silica [10], Nafion/zirconium phosphate [11,12], Nafion/Cs+ ions [13], Nafion–TiO2 nanoparticles [14], Nafion/sulfonated montmorillonite composite [15], blending the sulfonated poly(etheretherketone) (SPEEK) with Nafion solution [16]. However, the incorporation of inorganic nanopar-
∗ Corresponding authors. Tel.: +86 10 82339539; fax: +86 10 82339539. E-mail addresses: [email protected] (Y. Xiang), [email protected] (S. Lu). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.11.049
ticles such as sulfonated montmorillonite, cesium ions and SiO2 inevitably alters the microstructure of Nafion, resulting in the decay in mechanical properties and proton conductivity, especially in the case of using non-conducting inorganic particles. The heteropoly acids (HPAs) are super ionic proton conductors in their fully hydrated states [17,18]. Among the Keggin-type HPAs, phosphotungstic acid (H3 PW12 O4 , HPW) is the strongest acid and has the highest conductivity. A number of research studies have focused on the application of HPW in fuel cells [19–24]. These studies have shown that the cell output and stability greatly decay owing to the leakage of HPW in water [25,26]. In our previous work, a layer-by-layer self-assembly technique was introduced to trap and anchor HPW molecules, which serve as a stable proton conductor on Nafion surface [27]. It was widely accepted that Nafion has a dual structure with both hydrophobic and hydrophilic domain [28]. Methanol permeates primarily through the hydrophilic water-rich domain where the –SO3 − associated. Consequently, it would possibly be an efficient way for methanol-blocking to decrease the size of methanol diffusion channel. In this study, we aim to anchor HPW molecules in Nafion skeleton through the interaction between Cs+ and HPW. The immoblized Cs–HPW clusters is expected not only suppress methanol crossover but also maintain or enhance proton conductivity of the membrane. Owing to the enhanced multi-performance in terms of both methanol-blocking and conductivity, a better cell performance of DMFC based on the composited membrane could be expected than that of pristine Nafion.
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2. Experimental 2.1. Materials and reagents Cesium carbonate (Cs2 CO3 , MW = 325.82) and H3 PW12 O40 (HPW) were purchased from Aldrich–Sigma and used without any treatment. Nafion solution (5 wt%) and Nafion membrane were obtained from DuPont. Sulfuric acid (95–97 wt%), hydrogen peroxide (30 wt%), and methanol were obtained from Fluka. Milli-Q water (18.2 M cm at 25 ◦ C, Millipore) was used in the experiments. Nafion-212 membranes was treated according to the standard procedure previously described [29]: 30 min in a 5 wt% H2 O2 solution at 80 ◦ C, then 30 min in Milli-Q water at 80 ◦ C, and finally 30 min in an 8 wt% H2 SO4 solution at 80 ◦ C. After every treatment step, the membrane was rinsed with Milli-Q water three times to remove traces of H2 O2 or H2 SO4 . The membranes were stored in Milli-Q water before use.
to ensure uniformity. An Agilent GC 6890N with an HP-PLOT/U column was used for methanol analysis, and the peak areas were converted to methanol concentration by using a calibration curve. The methanol diffusion coefficient was obtained by analyzing the methanol flux with time and calculated as shown below: CB (t) =
APCA (t − t0 ) VB L
(1)
where CB and CA are the methanol concentration of permeated and feed side through the membrane, respectively. A, L and VB are the effective area of membrane, the thickness and the volume of permeated compartment, respectively. P is the methanol diffusion coefficient. And the stability of methanol-blocking property was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h, then the diffusion coefficient of the Cs–HPW–Nafion membrane was measured as a function of the time for which the membranes were immersed in Milli-Q water (immersion time).
2.2. Preparation of composite membranes The preparation of Cs–HPW–Nafion was carried out by the following procedures: pretreated Nafion-212 was immersed into the Cs2 CO3 solution (1 mM) at 60 ◦ C for 24 h with continuous stirring to substitute H+ with Cs+ . The cesium-substituted membrane was labeled as Cs–Nafion. To anchor HPW molecules in the bulk phase of the membrane, Cs–Nafion membrane was treated with HPW solution (20 mM) under the same conditions. The membrane was then labeled as Cs–HPW–Nafion. After each step, both Cs–Nafion and Cs–HPW–Nafion were rinsed thoroughly and immersed in Milli-Q water. 2.3. Characterization of composite membrane 2.3.1. UV–vis spectra determination The UV–vis spectra of all membranes were performed on GBC Cintra 10e instrument with scanning range from 190 nm to 800 nm. The scanning speed was 1000 nm/min and slit width was 1.5 nm. The stability of the Cs–HPW clusters in Nafion was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h, and the UV–vis spectra were measured as a function of immersion time.
2.3.5. Cell performance test To evaluate the performance of DMFCs, a membrane-electrodeassembly (MEA) was constructed by sandwiching a membrane between a PtRu (PtRu black, E-TEK) anode and a Pt/C (E-TEK, 40%) cathode, respectively. The loading of PtRu was 4 mg cm−2 for the anode and the loading of Pt was 0.4 mg cm−2 for the cathode. The Nafion resin in catalyst layer was 15 wt% for the anode and 25 wt% for the cathode. Carbon paper treated with poly(tetrafluoroethylene) was used as the gas diffusion layer. The sample was hot-pressed at 135 ◦ C for 60 s to form MEA. Methanol (2 mol L−1 ) was fed to the anode at a flow rate of 0.5 mL min−1 . Pure oxygen was supplied to the cathode at 100 mL min−1 at room temperature. Cell tests were carried out at room temperature with no back pressure. Besides, the limiting oxidation current of methanol from anode to cathode was determined to reflect the methanolblocking properties of the membranes during cell test [31,32]. In details, pure nitrogen was applied to take the place of oxygen in the cathode and methanol solutions was fed to anode with the same flux as the cell test. The linear sweep voltammetry was carried out when the OCV reduced to a stable potential. The scan range was 0.2–1 V with scan rate of 2 mV s−1 . 3. Results and discussion
2.3.2. Morphology and energy dispersive X-ray (EDX) analysis Both the surface and cross section morphology of Cs–HPW–Nafion membranes were examined under a scanning electron microscope equipped with EDS detector (SEM, Oxford, Camscan 3400; Oxford Instruments). Elemental mapping analysis was used to study the distribution of typical elements in the structure of composite membrane. 2.3.3. Conductivity measurements The through-plane proton conductivity of the membranes was measured using PARSTAT 2273 potentiostat in a frequency range of 0.1 Hz to 100 KHz with an amplitude of 10 mV at room temperature. The stability of the Cs–HPW clusters in Nafion was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h. Next, the conductivity of the Cs–HPW–Nafion membrane was measured as a function of the time for which the membranes were immersed in Milli-Q water (immersion time). 2.3.4. Methanol permeability measurement Methanol permeability was determined by using a diffusion cell method [30]. Methanol solution (2 mol L−1 ) and water were placed on different compartments of the diffusion cell and separated by the membrane. Magnetic stirrers were used in each compartment
Fig. 1 shows the scheme of HPW molecules anchored in the bulk structure of Nafion. Owing to the presence of H+ and the negatively charged sulfonic acid group, –SO3 − , in the Nafion membrane, the Cs+ ions first substitute H+ ion in Nafion to form Cs–Nafion by the ion-exchange method. HPW molecules are then trapped with Cs+ through an interaction and tightly immobilized in the Nafion. The HPW Keggin unit contains a three negative charges at the exterior of the structure, which is neutralized by three protons in the acid form at the exterior of the structure. In Cs–HPW–Nafion, except one negative charge that combined with Cs+ , the remained two negative charges combine with H+ to enhance the conductivity of the Cs–HPW–Nafion membrane. In addition, the dual structure of Nafion results in Cs–HPW clusters formed and homogeneously distributed in the water-rich domain of Cs–HPW–Nafion. The size of the methanol diffusion channel decreased clearly, which means that the Cs–HPW clusters act as a diffusion barrier for methanol transport. 3.1. UV–vis spectra determination Fig. 2 shows the UV–vis spectra of the membranes before and after HPW molecules were anchored. The absorbance exhibits a characteristic absorption peak at 265 nm, which increased sig-
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243
Fig. 1. Scheme of anchored HPW molecule in Nafion ion-cluster.
in the multi-structure was completely stable, and no leakage had occurred during the test procedure. 3.2. Scanning electron microscopy and elemental mapping analysis Fig. 3a shows the cross-section mophology of the Cs–HPW–Nafion composite membrane and indicates that there were no obvious structure transformations during the procedure. Fig. 3b shows the distribution of tungsten (W), a typical elements of Cs–HPW cluster in cross section of Cs–HPW–Nafion composite membrane. The dual structure of Nafion results in a homogeneous distribution of the Cs–HPW clusters in the water-rich domain, which contains the methanol diffusion channel.
Fig. 2. UV–vis spectra of the membranes before and after HPW anchored, and the inset plot is absorbance at 265 nm of Cs–HPW–Nafion as function of soaking time in Milli-Q water.
nificantly after the modification procedure. Thus, the absorbance peak at 265 nm was attributed to the adsorption of HPW in the Cs–HPW clusters [33]. The results indicated that HPW molecules were incorporated in Nafion. To evaluate the stability of HPW in the Cs–HPW–Nafion membrane, the composite membrane was immersed in Milli-Q water, and the UV spectra were measured as a function of immersion time. Immersion of the membrane in water for 5 h, 10 h, 25 h, and 30 h did not cause any obvious decay in the absorbance at 265 nm (Fig. 2, inset), which indicated that the HPW
3.3. Multi-performance Fig. 4 shows the proton conductivity of Nafion, Cs–Nafion and Cs–HPW–Nafion. After HPW immobilization, Cs–HPW–Nafion showed acceptable conductivity compared with unmodified Nafion under the same test conditions. Cs–HPW–Nafion had a conductivity of 3.6 × 10−2 S cm−1 , which was much higher than that of Cs–Nafion. Loss of H+ caused Cs–Nafion to yield a rather high resistance and an unacceptably low conductivity of 0.20 × 10−2 S cm−1 . To evaluate the stability of Cs–HPW clusters anchored in Nafion, the sample was soaked in Milli-Q water, and both in situ proton conductivity and methanol permeability were measured as a function of time.
Fig. 3. The cross section of Cs–HPW–Nafion (a) and mapping analysis of W on the selected area of cross section (b).
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Fig. 4. The conductivity of Cs–HPW–Nafion, Cs–Nafion and Nafion as a function of immersing time.
In cases where HPW molecules leaked from the Cs–HPW clusters, conductivity would change because of the decay of Cs–HPW–Nafion to Cs–Nafion. However, the stability of proton conductivity indicated that Cs–HPW–Nafion was rather stable in a water system and that the combined clusters did not leach out, probably due to strong electrostatic forces between Nafion and Cs–HPW clusters. Fig. 5 shows the comparison of the methanol permeability of Nafion-212 and Cs–HPW–Nafion. The methanol diffusion coefficient of Nafion was rather high, up to 8.35 × 10−6 cm2 s−1 . After modification, the methanol diffusion coefficient of Cs–HPW–Nafion decreased to 0.97 × 10−6 cm2 s−1 . The Obvious improvement in methanol-blockinng property results mainly from the generation of Cs–HPW clusters in water-rich domain of Nafion, leading to a decrease in the size of methanol diffusion channel. As shown in Fig. 5, the diffussion coefficient of Cs–HPW–Nafion composite membrane was as stable as a constant, which indicated Nafion obtained stable methanol-blocking property. In other words, the result indicated that Cs–HPW in the composite membrane was rather stable in a water system and that the combined clusters did not leach out. Since the performance of DMFC is affected by both proton conductivity and methanol permeability of the proton exchange membrane, the ratio of conductivity () to methanol permeability (P), /P, is an important selective factor for proton exchange membranes used in DMFCs [34]. A higher /P value indicates higher conductivity and lower methanol permeability and, thus, better cell performance would be expected. As shown in Fig. 6, the /P value of Cs–HPW–Nafion (3. 6 × 104 S cm−3 s) is clearly higher than that of Nafion (0.6 × 104 S cm−3 s), which indicated that after Cs–HPW modification procedure, Cs–HPW–Nafion composite membrane would be more suitable for DMFCs application.
Fig. 5. The methanol permeability of Cs–HPW–Nafion and Nafion as a function of immersing time.
Fig. 6. The comparison of selective factor between Cs–HPW–Nafion and Nafion.
3.4. Cell performance The cell performance of DMFCs assembled with an unmodified Nafion-212 membrane and Cs–HPW–Nafion is shown in Fig. 7a. At room temperature, the cell performance based on Cs–HPW–Nafion composite membrane in terms of power density was about 24 mW cm−2 , which is an increase of 26% over the cell performance based on pristine Nafion-212 membrane (19 mW cm−2 ). Clearly, the higher cell performance based on Cs–HPW–Nafion composite membrane was attributed to the fact that the modified membrane has lower methanol permeability than the pristine Nafion-212 membrane. Fig. 7b shows the methanol-blocking effect of the Cs–HPW–Nafion detected using the limiting current density during the cell test. In comparison the methanol crossover limiting current density curve with an unmodified Nafion-212 membrane is also shown in the figure. The methanol crossover limiting current density of Cs–HPW–Nafion membrane was 45.8 mA cm−2 , much lower than that of pristine Nafion-212 (169.8 mA cm−2 ) at room temperature. This indicated the reduction of methanol crossover of the Nafion membrane by about 30% on the Cs–HPW–Nafion membrane.
Fig. 7. The cell performance (a) and the crossover methanol oxidation reaction (MOR) limiting current density (b) of Cs–HPW–Nafion and Nafion.
Y. Xiang et al. / Journal of Membrane Science 368 (2011) 241–245
The reduction in the methanol crossover current demonstrates the potential of the efficient blocking of the methanol crossover by anchored HPW in the bulk phase of Nafion. The methanol crossover test results using the limiting current method are in agreement with the results using diffusion cell method in this study. 4. Conclusion In conclusion, a novel Cs–HPW–Nafion membrane was successfully prepared and was demonstrated to have a much higher performance in methanol-blocking and cell output. The results of UV–vis, EDX elemental mapping and conductivity stability test indicated that HPW successfully anchored in the bulk structure of Nafion and that the immobilized HPW was very stable. DMFC single cell test indicated that Cs–HPW–Nafion yielded a rather higher output than pristine Nafion-212 under the same experimental conditions. Thus, our results demonstrate the promising potential of the Cs–HPW–Nafion composite membrane as an effective PEM with lower fuel-leaking for application in DMFCs. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grants 20773008, 21003007, 21073010), National High-tech R&D Program (863 program, 2007AA05Z146), Beijing Novel Program (No. 2008B12) and the Fundamental Research Funds for the Central Universities (YWF-10-02-004). The authors are grateful to Dr. Dawei Liang (School of Chemistry and Environment, Beihang University) for correcting the language error in the manuscript. References [1] T. Satoshi, O. Sousuke, O. Hiroyuki, et al., On-chip fuel cell: micro direct methanol fuel cell of an air-breathing, membraneless, and monolithic design, J. Am. Chem. Soc. 130 (2008) 10456–10457. [2] X.Y. Zhang, W. Lu, J.Y. Da, et al., Porous platinum nanowire arrays for direct ethanol fuel cell applications, Chem. Commun. 2 (2009) 195–197. [3] R.H. Wang, C.G. Tian, L. Wang, et al., Electrochemical assay of superoxide based on biomimetic enzyme at highly conductive TiO2 nanoneedles: from principle to applications in living cells, Chem. Commun. 21 (2009) 3104–3106. [4] H.L. Tang, M. Pan, Synthesis and characterization of a self-assembled nafion/silica nanocomposite membrane for polymer electrolyte membrane fuel cells, J. Phys. Chem. C 112 (2008) 11556–11568. [5] H.L. Tang, P.K. Shen, S.P. Jiang, et al., A degradation study of Nafion proton exchange membrane of PEM fuel cells, J. Power Sources 170 (2007) 85–92. [6] Y. Xiang, M. Yang, Z.B. Guo, et al., Alternatively chitosan sulfate blending membrane as methanol-blocking polymer electrolyte membrane for direct methanol fuel cell, J. Membr. Sci. 337 (2009) 318–323. [7] Z.Q. Ma, P. Cheng, T.S. Zhao, A palladium-alloy deposited Nafion membrane for direct methanol fuel cells, J. Membr. Sci. 215 (2003) 327–336. [8] J. Prabhuram, T.S. Zhao, Z.X. Liang, et al., Pd and Pd–Cu alloy deposited nafion membranes for reduction of methanol crossover in direct methanol fuel cells, J. Electrochem. Soc. 152 (2005) 1390–1397. [9] N. Jia, M.C. Lefebvre, J. Halfyard, et al., Modification of Nafion proton exchange membranes to reduce methanol crossover in PEM fuel cells, Electrochem. Solid State Lett. 3 (2000) 529–531. [10] N. Miyake, J.S. Wainright, R.F. Savinell, Evaluation of a sol–gel derived Nafion/silica hybrid membrane for polymer electrolyte membrane fuel cell
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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Phosphotungstic acid (HPW) molecules anchored in the bulk of Nafion as methanol-blocking membrane for direct methanol fuel cells Yan Xiang a,b,∗ , Meng Yang a,b , Jin Zhang a , Fei Lan a , Shanfu Lu a,∗ a b
School of Chemistry and Environment, Beihang University, Beijing 100191, China School of Materials Science and Engineering, Beihang University, Beijing 100191, China
a r t i c l e
i n f o
Article history: Received 20 August 2010 Received in revised form 14 November 2010 Accepted 17 November 2010 Available online 23 November 2010 Keywords: Phosphotungstic acid (HPW) Nafion modification Methanol-blocking Direct methanol fuel cell (DMFC)
a b s t r a c t In the present study, a novel composite methanol-blocking polymer electrolyte membrane was prepared by anchoring water-soluble phosphotungstic acid (HPW) in bulk phase of Nafion through interactions between HPW and Cs+ ions. Results of morphology and elemental mapping analysis indicated that this composite membrane had a dense structure with uniformly distributed Cs–HPW clusters. The composite membrane exhibited a rather low methanol diffusion coefficient (P, 0.97 × 10−6 cm2 s−1 ) and also the comparable conductivity (, 3.6 × 10−2 S cm−1 ) contrast with pristine Nafion (, 5.1 × 10−2 S cm−1 ). The selective factor (/P) of the composite membrane was six times higher than that of pristine Nafion. Single cell performance results showed that the maximum power density increased by 26% over the cell performance of pristine Nafion under the same conditions. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Direct methanol fuel cells (DMFCs) have attracted increasing attention as a promising highly efficient energy conversion power sources for electric vehicles and portable electronic devices due to their low environmental pollutant emissions, simple structural design and convenient fuel storage [1–3]. Proton exchange membrane (PEM) is the “heart” of the DMFC system. The state-of-the-art and most commercialized PEM, the Nafion series membrane, could hardly sustain a satisfactory performance when applying in DMFC, since the methanol could readily migrate from the anode to the cathode through Nafion membrane. This migration results in a mixed potential, thus reducing the open-circuit voltage by as much as 0.15–0.2 V and poisoning the electrocatalysts at the cathode [4–6]. Thus, suppression of methanol crossover in Nafion has attracted intensive attention worldwide, and the approaches include membrane surface modification by depositing palladium and palladium alloys on Nafion membrane [7–9], and using composite membranes, such as sol–gel derived Nafion/silica [10], Nafion/zirconium phosphate [11,12], Nafion/Cs+ ions [13], Nafion–TiO2 nanoparticles [14], Nafion/sulfonated montmorillonite composite [15], blending the sulfonated poly(etheretherketone) (SPEEK) with Nafion solution [16]. However, the incorporation of inorganic nanopar-
∗ Corresponding authors. Tel.: +86 10 82339539; fax: +86 10 82339539. E-mail addresses: [email protected] (Y. Xiang), [email protected] (S. Lu). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.11.049
ticles such as sulfonated montmorillonite, cesium ions and SiO2 inevitably alters the microstructure of Nafion, resulting in the decay in mechanical properties and proton conductivity, especially in the case of using non-conducting inorganic particles. The heteropoly acids (HPAs) are super ionic proton conductors in their fully hydrated states [17,18]. Among the Keggin-type HPAs, phosphotungstic acid (H3 PW12 O4 , HPW) is the strongest acid and has the highest conductivity. A number of research studies have focused on the application of HPW in fuel cells [19–24]. These studies have shown that the cell output and stability greatly decay owing to the leakage of HPW in water [25,26]. In our previous work, a layer-by-layer self-assembly technique was introduced to trap and anchor HPW molecules, which serve as a stable proton conductor on Nafion surface [27]. It was widely accepted that Nafion has a dual structure with both hydrophobic and hydrophilic domain [28]. Methanol permeates primarily through the hydrophilic water-rich domain where the –SO3 − associated. Consequently, it would possibly be an efficient way for methanol-blocking to decrease the size of methanol diffusion channel. In this study, we aim to anchor HPW molecules in Nafion skeleton through the interaction between Cs+ and HPW. The immoblized Cs–HPW clusters is expected not only suppress methanol crossover but also maintain or enhance proton conductivity of the membrane. Owing to the enhanced multi-performance in terms of both methanol-blocking and conductivity, a better cell performance of DMFC based on the composited membrane could be expected than that of pristine Nafion.
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2. Experimental 2.1. Materials and reagents Cesium carbonate (Cs2 CO3 , MW = 325.82) and H3 PW12 O40 (HPW) were purchased from Aldrich–Sigma and used without any treatment. Nafion solution (5 wt%) and Nafion membrane were obtained from DuPont. Sulfuric acid (95–97 wt%), hydrogen peroxide (30 wt%), and methanol were obtained from Fluka. Milli-Q water (18.2 M cm at 25 ◦ C, Millipore) was used in the experiments. Nafion-212 membranes was treated according to the standard procedure previously described [29]: 30 min in a 5 wt% H2 O2 solution at 80 ◦ C, then 30 min in Milli-Q water at 80 ◦ C, and finally 30 min in an 8 wt% H2 SO4 solution at 80 ◦ C. After every treatment step, the membrane was rinsed with Milli-Q water three times to remove traces of H2 O2 or H2 SO4 . The membranes were stored in Milli-Q water before use.
to ensure uniformity. An Agilent GC 6890N with an HP-PLOT/U column was used for methanol analysis, and the peak areas were converted to methanol concentration by using a calibration curve. The methanol diffusion coefficient was obtained by analyzing the methanol flux with time and calculated as shown below: CB (t) =
APCA (t − t0 ) VB L
(1)
where CB and CA are the methanol concentration of permeated and feed side through the membrane, respectively. A, L and VB are the effective area of membrane, the thickness and the volume of permeated compartment, respectively. P is the methanol diffusion coefficient. And the stability of methanol-blocking property was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h, then the diffusion coefficient of the Cs–HPW–Nafion membrane was measured as a function of the time for which the membranes were immersed in Milli-Q water (immersion time).
2.2. Preparation of composite membranes The preparation of Cs–HPW–Nafion was carried out by the following procedures: pretreated Nafion-212 was immersed into the Cs2 CO3 solution (1 mM) at 60 ◦ C for 24 h with continuous stirring to substitute H+ with Cs+ . The cesium-substituted membrane was labeled as Cs–Nafion. To anchor HPW molecules in the bulk phase of the membrane, Cs–Nafion membrane was treated with HPW solution (20 mM) under the same conditions. The membrane was then labeled as Cs–HPW–Nafion. After each step, both Cs–Nafion and Cs–HPW–Nafion were rinsed thoroughly and immersed in Milli-Q water. 2.3. Characterization of composite membrane 2.3.1. UV–vis spectra determination The UV–vis spectra of all membranes were performed on GBC Cintra 10e instrument with scanning range from 190 nm to 800 nm. The scanning speed was 1000 nm/min and slit width was 1.5 nm. The stability of the Cs–HPW clusters in Nafion was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h, and the UV–vis spectra were measured as a function of immersion time.
2.3.5. Cell performance test To evaluate the performance of DMFCs, a membrane-electrodeassembly (MEA) was constructed by sandwiching a membrane between a PtRu (PtRu black, E-TEK) anode and a Pt/C (E-TEK, 40%) cathode, respectively. The loading of PtRu was 4 mg cm−2 for the anode and the loading of Pt was 0.4 mg cm−2 for the cathode. The Nafion resin in catalyst layer was 15 wt% for the anode and 25 wt% for the cathode. Carbon paper treated with poly(tetrafluoroethylene) was used as the gas diffusion layer. The sample was hot-pressed at 135 ◦ C for 60 s to form MEA. Methanol (2 mol L−1 ) was fed to the anode at a flow rate of 0.5 mL min−1 . Pure oxygen was supplied to the cathode at 100 mL min−1 at room temperature. Cell tests were carried out at room temperature with no back pressure. Besides, the limiting oxidation current of methanol from anode to cathode was determined to reflect the methanolblocking properties of the membranes during cell test [31,32]. In details, pure nitrogen was applied to take the place of oxygen in the cathode and methanol solutions was fed to anode with the same flux as the cell test. The linear sweep voltammetry was carried out when the OCV reduced to a stable potential. The scan range was 0.2–1 V with scan rate of 2 mV s−1 . 3. Results and discussion
2.3.2. Morphology and energy dispersive X-ray (EDX) analysis Both the surface and cross section morphology of Cs–HPW–Nafion membranes were examined under a scanning electron microscope equipped with EDS detector (SEM, Oxford, Camscan 3400; Oxford Instruments). Elemental mapping analysis was used to study the distribution of typical elements in the structure of composite membrane. 2.3.3. Conductivity measurements The through-plane proton conductivity of the membranes was measured using PARSTAT 2273 potentiostat in a frequency range of 0.1 Hz to 100 KHz with an amplitude of 10 mV at room temperature. The stability of the Cs–HPW clusters in Nafion was evaluated at room temperature by immersing the Cs–HPW–Nafion membrane in Milli-Q water and changing the water every 12 h. Next, the conductivity of the Cs–HPW–Nafion membrane was measured as a function of the time for which the membranes were immersed in Milli-Q water (immersion time). 2.3.4. Methanol permeability measurement Methanol permeability was determined by using a diffusion cell method [30]. Methanol solution (2 mol L−1 ) and water were placed on different compartments of the diffusion cell and separated by the membrane. Magnetic stirrers were used in each compartment
Fig. 1 shows the scheme of HPW molecules anchored in the bulk structure of Nafion. Owing to the presence of H+ and the negatively charged sulfonic acid group, –SO3 − , in the Nafion membrane, the Cs+ ions first substitute H+ ion in Nafion to form Cs–Nafion by the ion-exchange method. HPW molecules are then trapped with Cs+ through an interaction and tightly immobilized in the Nafion. The HPW Keggin unit contains a three negative charges at the exterior of the structure, which is neutralized by three protons in the acid form at the exterior of the structure. In Cs–HPW–Nafion, except one negative charge that combined with Cs+ , the remained two negative charges combine with H+ to enhance the conductivity of the Cs–HPW–Nafion membrane. In addition, the dual structure of Nafion results in Cs–HPW clusters formed and homogeneously distributed in the water-rich domain of Cs–HPW–Nafion. The size of the methanol diffusion channel decreased clearly, which means that the Cs–HPW clusters act as a diffusion barrier for methanol transport. 3.1. UV–vis spectra determination Fig. 2 shows the UV–vis spectra of the membranes before and after HPW molecules were anchored. The absorbance exhibits a characteristic absorption peak at 265 nm, which increased sig-
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243
Fig. 1. Scheme of anchored HPW molecule in Nafion ion-cluster.
in the multi-structure was completely stable, and no leakage had occurred during the test procedure. 3.2. Scanning electron microscopy and elemental mapping analysis Fig. 3a shows the cross-section mophology of the Cs–HPW–Nafion composite membrane and indicates that there were no obvious structure transformations during the procedure. Fig. 3b shows the distribution of tungsten (W), a typical elements of Cs–HPW cluster in cross section of Cs–HPW–Nafion composite membrane. The dual structure of Nafion results in a homogeneous distribution of the Cs–HPW clusters in the water-rich domain, which contains the methanol diffusion channel.
Fig. 2. UV–vis spectra of the membranes before and after HPW anchored, and the inset plot is absorbance at 265 nm of Cs–HPW–Nafion as function of soaking time in Milli-Q water.
nificantly after the modification procedure. Thus, the absorbance peak at 265 nm was attributed to the adsorption of HPW in the Cs–HPW clusters [33]. The results indicated that HPW molecules were incorporated in Nafion. To evaluate the stability of HPW in the Cs–HPW–Nafion membrane, the composite membrane was immersed in Milli-Q water, and the UV spectra were measured as a function of immersion time. Immersion of the membrane in water for 5 h, 10 h, 25 h, and 30 h did not cause any obvious decay in the absorbance at 265 nm (Fig. 2, inset), which indicated that the HPW
3.3. Multi-performance Fig. 4 shows the proton conductivity of Nafion, Cs–Nafion and Cs–HPW–Nafion. After HPW immobilization, Cs–HPW–Nafion showed acceptable conductivity compared with unmodified Nafion under the same test conditions. Cs–HPW–Nafion had a conductivity of 3.6 × 10−2 S cm−1 , which was much higher than that of Cs–Nafion. Loss of H+ caused Cs–Nafion to yield a rather high resistance and an unacceptably low conductivity of 0.20 × 10−2 S cm−1 . To evaluate the stability of Cs–HPW clusters anchored in Nafion, the sample was soaked in Milli-Q water, and both in situ proton conductivity and methanol permeability were measured as a function of time.
Fig. 3. The cross section of Cs–HPW–Nafion (a) and mapping analysis of W on the selected area of cross section (b).
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Fig. 4. The conductivity of Cs–HPW–Nafion, Cs–Nafion and Nafion as a function of immersing time.
In cases where HPW molecules leaked from the Cs–HPW clusters, conductivity would change because of the decay of Cs–HPW–Nafion to Cs–Nafion. However, the stability of proton conductivity indicated that Cs–HPW–Nafion was rather stable in a water system and that the combined clusters did not leach out, probably due to strong electrostatic forces between Nafion and Cs–HPW clusters. Fig. 5 shows the comparison of the methanol permeability of Nafion-212 and Cs–HPW–Nafion. The methanol diffusion coefficient of Nafion was rather high, up to 8.35 × 10−6 cm2 s−1 . After modification, the methanol diffusion coefficient of Cs–HPW–Nafion decreased to 0.97 × 10−6 cm2 s−1 . The Obvious improvement in methanol-blockinng property results mainly from the generation of Cs–HPW clusters in water-rich domain of Nafion, leading to a decrease in the size of methanol diffusion channel. As shown in Fig. 5, the diffussion coefficient of Cs–HPW–Nafion composite membrane was as stable as a constant, which indicated Nafion obtained stable methanol-blocking property. In other words, the result indicated that Cs–HPW in the composite membrane was rather stable in a water system and that the combined clusters did not leach out. Since the performance of DMFC is affected by both proton conductivity and methanol permeability of the proton exchange membrane, the ratio of conductivity () to methanol permeability (P), /P, is an important selective factor for proton exchange membranes used in DMFCs [34]. A higher /P value indicates higher conductivity and lower methanol permeability and, thus, better cell performance would be expected. As shown in Fig. 6, the /P value of Cs–HPW–Nafion (3. 6 × 104 S cm−3 s) is clearly higher than that of Nafion (0.6 × 104 S cm−3 s), which indicated that after Cs–HPW modification procedure, Cs–HPW–Nafion composite membrane would be more suitable for DMFCs application.
Fig. 5. The methanol permeability of Cs–HPW–Nafion and Nafion as a function of immersing time.
Fig. 6. The comparison of selective factor between Cs–HPW–Nafion and Nafion.
3.4. Cell performance The cell performance of DMFCs assembled with an unmodified Nafion-212 membrane and Cs–HPW–Nafion is shown in Fig. 7a. At room temperature, the cell performance based on Cs–HPW–Nafion composite membrane in terms of power density was about 24 mW cm−2 , which is an increase of 26% over the cell performance based on pristine Nafion-212 membrane (19 mW cm−2 ). Clearly, the higher cell performance based on Cs–HPW–Nafion composite membrane was attributed to the fact that the modified membrane has lower methanol permeability than the pristine Nafion-212 membrane. Fig. 7b shows the methanol-blocking effect of the Cs–HPW–Nafion detected using the limiting current density during the cell test. In comparison the methanol crossover limiting current density curve with an unmodified Nafion-212 membrane is also shown in the figure. The methanol crossover limiting current density of Cs–HPW–Nafion membrane was 45.8 mA cm−2 , much lower than that of pristine Nafion-212 (169.8 mA cm−2 ) at room temperature. This indicated the reduction of methanol crossover of the Nafion membrane by about 30% on the Cs–HPW–Nafion membrane.
Fig. 7. The cell performance (a) and the crossover methanol oxidation reaction (MOR) limiting current density (b) of Cs–HPW–Nafion and Nafion.
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The reduction in the methanol crossover current demonstrates the potential of the efficient blocking of the methanol crossover by anchored HPW in the bulk phase of Nafion. The methanol crossover test results using the limiting current method are in agreement with the results using diffusion cell method in this study. 4. Conclusion In conclusion, a novel Cs–HPW–Nafion membrane was successfully prepared and was demonstrated to have a much higher performance in methanol-blocking and cell output. The results of UV–vis, EDX elemental mapping and conductivity stability test indicated that HPW successfully anchored in the bulk structure of Nafion and that the immobilized HPW was very stable. DMFC single cell test indicated that Cs–HPW–Nafion yielded a rather higher output than pristine Nafion-212 under the same experimental conditions. Thus, our results demonstrate the promising potential of the Cs–HPW–Nafion composite membrane as an effective PEM with lower fuel-leaking for application in DMFCs. Acknowledgements This work was supported by the National Nature Science Foundation of China (Grants 20773008, 21003007, 21073010), National High-tech R&D Program (863 program, 2007AA05Z146), Beijing Novel Program (No. 2008B12) and the Fundamental Research Funds for the Central Universities (YWF-10-02-004). The authors are grateful to Dr. Dawei Liang (School of Chemistry and Environment, Beihang University) for correcting the language error in the manuscript. References [1] T. Satoshi, O. Sousuke, O. Hiroyuki, et al., On-chip fuel cell: micro direct methanol fuel cell of an air-breathing, membraneless, and monolithic design, J. Am. Chem. Soc. 130 (2008) 10456–10457. [2] X.Y. Zhang, W. Lu, J.Y. Da, et al., Porous platinum nanowire arrays for direct ethanol fuel cell applications, Chem. Commun. 2 (2009) 195–197. [3] R.H. Wang, C.G. Tian, L. Wang, et al., Electrochemical assay of superoxide based on biomimetic enzyme at highly conductive TiO2 nanoneedles: from principle to applications in living cells, Chem. Commun. 21 (2009) 3104–3106. [4] H.L. Tang, M. Pan, Synthesis and characterization of a self-assembled nafion/silica nanocomposite membrane for polymer electrolyte membrane fuel cells, J. Phys. Chem. C 112 (2008) 11556–11568. [5] H.L. Tang, P.K. Shen, S.P. Jiang, et al., A degradation study of Nafion proton exchange membrane of PEM fuel cells, J. Power Sources 170 (2007) 85–92. [6] Y. Xiang, M. Yang, Z.B. Guo, et al., Alternatively chitosan sulfate blending membrane as methanol-blocking polymer electrolyte membrane for direct methanol fuel cell, J. Membr. Sci. 337 (2009) 318–323. [7] Z.Q. Ma, P. Cheng, T.S. Zhao, A palladium-alloy deposited Nafion membrane for direct methanol fuel cells, J. Membr. Sci. 215 (2003) 327–336. [8] J. Prabhuram, T.S. Zhao, Z.X. Liang, et al., Pd and Pd–Cu alloy deposited nafion membranes for reduction of methanol crossover in direct methanol fuel cells, J. Electrochem. Soc. 152 (2005) 1390–1397. [9] N. Jia, M.C. Lefebvre, J. Halfyard, et al., Modification of Nafion proton exchange membranes to reduce methanol crossover in PEM fuel cells, Electrochem. Solid State Lett. 3 (2000) 529–531. [10] N. Miyake, J.S. Wainright, R.F. Savinell, Evaluation of a sol–gel derived Nafion/silica hybrid membrane for polymer electrolyte membrane fuel cell
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